The present invention relates to a photovoltaic element sequentially comprising at least an amorphous thin film semiconductive layer of one conductivity type and an amorphous thin film semiconductive layer of the other conductivity type on a surface of a transparent conductive film, and a method of manufacturing the photovoltaic element.
In a thin film photovoltaic element utilizing a photoelectric conversion characteristic of an amorphous thin film semiconductor such as amorphous silicon, amorphous silicon germanium or amorphous silicon carbide, the thickness of a semiconductive film required for light absorption can be more reduced than the photovoltaic element utilizing a crystalline semiconductor such as a monocrystalline silicon or a polycrystalline silicon. Therefore, the cost of the material of the element can be reduced.
For example, the structure of the photovoltaic element using the amorphous silicon will be described with reference to a sectional view showing the structure of the element in FIG. 1. A transparent conductive film 12 formed of a transparent conductive material such as SnO2, ITO or ZnO, a first amorphous thin film semiconductive layer 13 formed of a p-type amorphous silicon carbide film, a second amorphous thin film semiconductive layer 14 formed of an i-type amorphous silicon film, a third amorphous thin film semiconductive layer 15 formed of an n-type amorphous silicon film, and a back metal film 16 formed of Al or Ag are sequentially provided on a substrate 11 formed of a transparent insulator such as glass or plastic, thereby constituting the photovoltaic element.
In the photovoltaic element, light is incident from the substrate 11 side and electron-hole pairs generated by light absorption in the second amorphous thin film semiconductive layer 14 formed of the i-type amorphous silicon film are isolated by an electric field formed by a pin junction. The electron-hole pairs are collected through the back metal film 16 and the transparent conductive film 12 so that the photoelectromotive force is generated. In order to enhance the photoelectric conversion characteristic of the photovoltaic element, accordingly, it is necessary to increase the amount of light to be absorbed in the second amorphous thin film semiconductive layer 14. For this reason, there is utilized multiple reflection in which light is reflected by an interface between the third amorphous thin film semiconductive layer 15 and the back metal film 16 and an interface between the first amorphous thin film semiconductive layer 13 and the transparent conductive film 12, thereby causing light to pass the second amorphous thin film semiconductive layer 14 plural times, or concavo-convex shapes are formed on the light incidence side of the second amorphous thin film semiconductive layer 14, the back side or both sides thereof, thereby scattering the light. Thus, an optical path length is substantially extended. At the same time, it is necessary to reduce the light absorption in layers other than the second amorphous thin film semiconductive layer 14, that is, the substrate 11, the transparent conductive film 12, the first amorphous thin film semiconductive layer 13, the third amorphous thin film semiconductive layer 15 and the back metal film 16. In particular, it is important that the light absorption in the substrate 11, the transparent conductive film 12 and the first amorphous thin film semiconductive layer 13 present on the light incidence side of the second amorphous thin film semiconductive layer 14 should be suppressed.
The substrate 11 is formed of a transparent material such as glass so that the substrate 11 almost fully transmits light with a wavelength region having a light intensity with respect to the amorphous silicon film. Therefore, it is desirable that the light absorption in the transparent conductive film 12 and the first amorphous thin film semiconductive layer 13 should be reduced. For this reason, the amorphous silicon carbide is used for the first amorphous thin film semiconductive layer 13.
As described above, it is particularly necessary to reduce the light absorption in the transparent conductive film 12 and the first amorphous thin film semiconductive layer 13 in order to enhance the photoelectric conversion characteristic of the photovoltaic element.
The light absorption in the transparent conductive film 12 can be reduced by enhancing the crystalline property of a metal oxide such as SnO2, ITO or ZnO constituting the transparent conductive film 12. However, in the case in which the crystalline property of the metal oxide is enhanced, an ohmic property is deteriorated between the transparent conductive film 12 and the first amorphous thin film semiconductive layer 13 formed thereon. In particular, in the case in which the amorphous silicon carbide film is used as the first amorphous thin film semiconductive layer 13 in order to reduce the light absorption, the ohmic property is remarkably deteriorated between the amorphous silicon carbide film and the transparent conductive film 12 formed of a metal oxide having a high crystalline property. Consequently, an excellent photoelectric conversion characteristic cannot be obtained.
In order to form a high ohmic junction together with the transparent conductive film 12 formed of the metal oxide having a high crystalline property, an attempt to enhance the conductive property of the first amorphous thin film semiconductive layer 13 formed on the transparent conductive film 12 has been made. While an impurity element is intentionally added to the first amorphous thin film semiconductive layer 13 in order to enhance the conductive property, the conductive property of the first amorphous thin film semiconductive layer 13 can be enhanced by increasing the concentration of the impurity element. Moreover, there is also used a method of microcrystallizing the first amorphous thin film semiconductive layer 13. However, in the case in which the concentration of the impurity element in the first amorphous thin film semiconductive layer 13 is increased, a defect density in the film to be the recombination center of a carrier is increased so that the photoelectric conversion characteristic is deteriorated. Moreover, if the microcrystallized film is to be used as the first amorphous thin film semiconductive layer 13, a film thickness should be increased because the microcrystallization cannot be carried out with a thin film. As a result, the light absorption in the first amorphous thin film semiconductive layer 13 is increased so that the photoelectric conversion characteristic cannot be enhanced.
Moreover, there has also been known the fact that the ohmic property of the interface between the transparent conductive film formed of ZnO and the p-type amorphous semiconductor can be enhanced through a treatment on the surface of the transparent conductive film through a diborane plasma. However, hydrogen radical is generated in the diborane plasma by decomposition of a diborane gas and the hydrogen radical reduces a ZnO surface. The reduced surface increases the light absorption so that the photoelectric conversion characteristic is enhanced slightly.
It is an object of the present invention to provide a photovoltaic element capable of enhancing an ohmic property between a transparent conductive film and an amorphous semiconductive layer of one conductivity type (a first amorphous thin film semiconductive layer) to have an excellent photoelectric conversion characteristic and a method of manufacturing the photovoltaic element.
The present invention provides a photovoltaic element sequentially comprising at least an amorphous semiconductive layer of one conductivity type and an amorphous semiconductive layer of the other conductivity type on a surface of a transparent conductive film, wherein the transparent conductive film includes a surface region having a lower crystalline property on a surface side than that in an inner portion and the amorphous semiconductive layer of one conductivity type is formed on the surface region. Accordingly, the ohmic property between the transparent conductive film and the amorphous semiconductive layer of one conductivity type can be enhanced and an excellent photoelectric conversion characteristic can be obtained.
It is preferable that the surface region should have a thickness of 5 to 300 xc3x85. Moreover, it is preferable that the transparent conductive film should be formed of ZnO, SnO2 and ITO. Furthermore, in the case in which the amorphous semiconductive layer of one conductivity type is formed of amorphous silicon carbide, the present invention can produce remarkable effects.
Moreover, the present invention provides a method of manufacturing a photovoltaic element comprising the steps of processing a surface of a transparent conductive film through a rare-gas plasma and forming an amorphous semiconductive layer of one conductivity type on a surface region of the transparent conductive film processed by the rare-gas plasma. Thus, the rare-gas plasma process is carried out over the surface of the transparent conductive film. Consequently, the surface region having a low crystalline property can be formed easily.
The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.